Lightning rods, or, more particularly, “air terminals” or “finials,” are conductors that may be mounted on top of a building, tower, antenna, or other structure, and electrically connected to the ground through a wire, called a “down-conductor,” to intercept the charge that terminates on a structure and carry it to the ground. If lightning strikes the structure, it will preferentially strike the air terminal, and be conducted relatively harmlessly to ground through the down-conductor, instead of passing through the structure, where it could cause various forms of damage, such as fire or electrical damage to sensitive components.
Air terminals are generally designed to convey the current from a strike to the ground (or water) via a low-resistance down-conductor that is inserted into the ground, ideally in an area of high ground conductivity. While the electrical resistance of the lightning conductor between the air terminal and the earth is important, the inductance of the conductor is an important consideration as well. For this reason, it is known that it is best to keep the down-conductor route short and to ensure that any curves or bends in the down-conductor have a large radius. If these measures are not taken, lightning current is more likely to arc over an obstruction, resistive or reactive, that it encounters in the down-conductor. At the very least, the arcing may damage the lightning conductor and the current is divided among alternate conductive paths to ground, such as through the structure (building wiring/plumbing, metal structural components, etc.) and can cause unwanted damage.
Often, structures that require lightning strike protection are adjustable in height, such as telescoping towers, masts, antennas, etc. Because the structure/payload is at an indeterminate height above the earth, proper bonding for lightning strike mitigation is inherently difficult, leading to excessive lengths of grounding cables which result in performance degradation. As noted above, it is best to avoid kinks or coils in a down-conductor for lightning strikes to reduce the impedance seen by the lightning to reach the ground.
Various conventional solutions to this problem have been utilized to account for the variable-length of the structure and associated air terminal. For instance, in one technique, the excess grounding cable is coiled at the base of the structure next to a grounding rod inserted into the earth. However, the coils and sharp bends in the lightning strike down-conductor (grounding) cable are undesirable as they increase the impedance seen by the high frequency event. Alternatively, in another technique, grounding cables can also be deployed away from the structure to a ground rod in an effort to “use up” excess cable length (avoiding loops and sharp turns). As the adjustable-height structure is lowered, however, an intermediate attachment point or separate ground rod needs to be driven to accommodate this change in the amount of excess cable length. In this technique in particular, the down-conductor is manually deployed, thus potentially endangering an operator due to height adjustments in response to lightning activity, in addition to the general inconvenience of the operation itself.